Problem: The scale on a map is 3cm : 3km. If the distance between two cities is 15km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 3cm on the map corresponds to an actual distance of 3km. An actual distance of 15km is the same as 5 $\cdot$ 3km. The distance between the two cities on the map is 5 $\cdot$ 3cm, or 15cm.